How free are we in what we do? Are our actions already predetermined in the brain long before we make a conscious decision? The Libet experiment from the year 1983, which suggests these conclusions, is still regarded today as the most important experiment on free will. Now a team of scientists at the Medical Center – University of Freiburg has presented an alternative explanation for the experiment and demonstrated that there is no contradiction between the principle of free will and previous neurobiological experiments. Their full explanation, supported by results from several studies conducted in recent years, has now been published on 14 July, 2016 as advanced online publication in the journal Neuroscience & Biobehavioral Reviews.
In 1983 the physiologist Benjamin Libet conducted an experiment that is still seen today as a landmark in brain research on the concept of free will. The test subjects were asked to make a spontaneous hand movement and then specify the moment at which they had decided to make it. Since the subjects were looking at a quickly moving clock during the experiment, they were able to name the precise instant at which they had made their decision. This instant was approximately 200 milliseconds before the movement itself began. One second beforehand, however, the researchers were already able to measure a specific brain signal, the readiness potential. The signal began approximately one to 1.5 seconds before the movement and increased steadily, before reaching its peak when the movement occurred. Libet and many others interpreted the finding as evidence that the subject’s belief of having made a deliberate decision to make the movement was an illusion, because the brain had already been preparing the action far in advance.
Brain Signal Facilitates Decisions but Does not Trigger Them
Now a team of researchers led by Prof. Dr. Stefan Schmidt, psychologist at the University of Freiburg Medical Center’s Department of Psychosomatic Medicine and Psychotherapy, has presented an alternative explanation that is consistent with the principle of free will. Contrary to the classical interpretation, they see the increase in readiness potential not as the cause of the decision and the action but as an accompanying phenomenon.
The early readiness potential up to approximately 400 to 500 milliseconds before the beginning of the action is presumably the result of very slow background fluctuations in brain activity. When these slow cortical potentials swing into the negative range, the brain evidently becomes more reactive: reaction times become shorter, perception more sensitive. As the researchers succeeded in demonstrating, test subjects undergoing the Libet experiment are also more likely to decide to make a spontaneous movement in these negative phases.
Since the Libet experiment is generally repeated many times and the mean results are taken as the basis for evaluation, these negative fluctuations add up and lead to the readiness potential. “We know from the experiments that a negative readiness potential facilitates decisions but does not trigger them. It is one of many influencing factors,” says Prof. Schmidt.
By conducting the Libet experiment themselves, the researchers proved in 2013 that the link between the readiness potential and the decision is much weaker than previously thought. Contrary to the usual practice, however, they evaluated each repetition of the experiment individually rather than taking the mean of up to 40 repetitions. It turned out that the brain signal was not negative but positive or neutral in one-third of the repetitions. “That contradicts the common assumption that the increase constitutes a direct preparation of the action,” says Prof. Schmidt. The scientists integrated all of these findings into their slow cortical potential sampling hypothesis, the SCP hypothesis for short.
Experienced Meditators Can Control the Impulse to Act
The researchers also have an explanation for why most decisions are made while the slow fluctuations are in the negative range. “The subjects evidently feel the increase in readiness potential as an inner impulse or a need to make a decision on carrying out the action,” says Prof. Schmidt.
The researchers also carried out the experiment several times on test subjects with experience in meditation. Due to their ability to stabilize attention, people who meditate are better than those who do not at observing and reporting on mental processes. A meditation master succeeded in reliably identifying the inner impulse to act, and thus also the negative fluctuation. When he followed the impulse, the readiness potential became stronger as expected. When he acted without an impulse, it became weaker. When he delayed the action following the impulse, the readiness potential was also delayed accordingly. “We are not determined by the readiness potential alone; we can even consciously change it,” says Prof. Schmidt.
Original title of the study: “Catching the Waves” − Slow Cortical Potentials as Moderator of Voluntary Action
Prof. Dr. Stefan Schmidt
Section Head: Complementary Medicine Evaluation Research
Department of Psychosomatic Medicine and Psychotherapy
Medical Center – University of Freiburg
Phone: +49 (0)761 270-69280